Need to fit a ferroli combi for a customer, the only thing is the combi also goes through two zone valves to go to central heating and the other goes to a cylinder for more water for a bath etc, the normal hot diverter being for a different part of the house,it all goes into a wiring center with the room stat which is mains voltage, the new boiler is a 24 and under voltage for the circuit board, so it needs zero voltage switching, what should happen as the stat for the boiler is two wire with a simple loop, I rang Ferroli and the said I need a relay to go into the boiler to drop the voltage, dont exactly know what they are talking about as the stat wiil stay in the wiring center I presume as its mains, so what do I do about boiler over run for the pump to disipate the heat, any help would be appreciated also what exactly is this thing part number39821670 which comes up as a six pin relay sold in a pack of five, Thanks.

so confused, October 2009

This won't work if you intend to use an internal programmer because the Cylinder DHW needs to heat independent of the heating.
I'm not a plumber so you will have to ensure that plumbing things are OK such as sufficient bypass for pump overrun etc.
The MVs need to be suitable for an S plan.

1) Rewire the existing system to a Honeywell S plan if it is not already. Include thermostats and a Htg & DHW programmer. The DHW circuit will operate the cylinder. The hot water from the combi will work as normal.
2) Instead of connecting to a boiler, connect the coil of a 230v relay with a "Normally Open" (N.O.) contact rating of at least 5 amps.
3)Wire the boiler as per the manufacturer's instructions from the same power source as the S plan. 4)Connect (one of) the normally open (N.O.) contact pairs of the relay to the thermostat connections.

You may need to mount the relay in an enclosure near the wiring centre.
You can get a 230/240v relay from internet, somewhere like CPC, RS components, or Farnell components. It needs to last many years of constant use, so keep away from the cheapest of the cheap. It can have any number of contacts as long as there is at least one N.O. pair. The type that use a plug in base with screw terminals are the easiest to install, but need a larger enclosure. Don't forget to order the base with the relay.
Note that when both DHW and CH are satisfied the MVs will both close and the boiler will shut down. A suitable bypass for pump overrun will be needed.

Steve, November 2009